There are known honeycomb structural bodies, in which catalyst is supported, capable of purifying exhaust gas emitted from an internal combustion engine of motor vehicles, etc. For example, the honeycomb structural body is comprised of partition walls arranged in a lattice shape and a plurality of cells formed by the partition walls. The honeycomb structural body is mounted to an exhaust gas passage through which exhaust gas is passing. When exhaust gas having a high temperature flows through the honeycomb structural body mounted to the exhaust gas passage, catalyst supported in the honeycomb structural body is activated by the exhaust gas having a high temperature. The activated catalyst purifies the exhaust gas.
Recently, because vehicles emission control for reducing motor vehicle emissions becomes stricter year by year, there is a strong demand for further decreasing harmful materials (cold emissions) generated immediately when an internal combustion engine starts. In order to solve this problem, it is required to use a technique for decreasing a thickness of each of the partition walls to decrease an overall weight of a honeycomb structural body and speedily increase a temperature of the honeycomb structural body to a target temperature in order to activate the catalyst. It is further required to use a technique for providing exhaust gas having a uniform flow speed and distribution to a honeycomb structural body in order to effectively use an overall catalyst supported by the honeycomb structural body. For this reason, it is requested for the honeycomb structural body to have a superior performance of purifying exhaust gas and a low pressure loss (also imply referred to as pressure loss), etc.
For example, a patent document 1 has disclosed a honeycomb structural body having a central section and an outer peripheral section in a cross section which is perpendicular to an axial direction of the honeycomb structural body. The patent document 1 has determined a relationship in a catalyst amount, a surface area, a cell density, etc. between the central section and the outer peripheral section. Further, a patent document 2 has disclosed a structure of a honeycomb structural body comprised of an inside peripheral wall having a cylindrical shape, first cells and second cells. In the structure, the first cells have a hexagonal shape arranged in the inside of the inner circumferential wall. The second cells have a shape (a circle, an ellipse, a square and a triangle) different from a hexagonal shape and arranged in the outside of the inner circumferential wall. In addition, a patent document 3 has disclosed a honeycomb structural body comprised of cells having a square shape, and a relationship in an opening ratio and an absorbing ratio, etc. between a central section and an outer peripheral section in the honeycomb structural body.
A patent document 4 has disclosed a honeycomb structural body having a structure in which an inside skin section is formed between a first cell section (as a central section) and a second cell section (as an outer peripheral section). The patent document 4 has determined a relationship in a cell density, etc. between the first cell section and the second cell section. Further, a patent document 5 has disclosed a honeycomb structural body having a structure in which web frames (i.e. partition walls) are formed from a central section toward an outer peripheral section, and the number of the web frames is changed in a radial direction of the honeycomb structural body. Still further, a patent document 6 has disclosed a structure of a honeycomb structural body in which partition walls have a shape protruded from a central point toward an outside, and a cell density of a central section is smaller than a cell density of the outer peripheral section. Still further, a patent document 7 has disclosed a structure of the honeycomb structural body in which an opening ratio of cells is increased sequentially or stepwise from a central section to an outer peripheral section.
Further, a patent document 8 has disclosed a honeycomb structural body having a structure in which a first honeycomb section (as a central section) and a second honeycomb section (as an outer peripheral section) is directly assembled together to form a monolithic mold, and the first honeycomb section and the second honeycomb section have a different cell density, a different thickness of a partition wall, a different shape of a cross section of cells, etc. Still further, a patent document 9 has disclosed a honeycomb structural body comprised of a plurality of honeycomb segments assembled together to form a monolithic body. The patent document 9 has determined a relationship in a thickness of a partition wall, a cell density, etc. between the outermost honeycomb segment, which forms an outermost peripheral surface, and the other honeycomb segments which do not form the outermost peripheral surface. Furthermore, a patent document 10 has disclosed a honeycomb structural body having a section in which a cell density is gradually decreased from a center toward an outer periphery.